In the tires of heavy duty type, the carcass reinforcement is generally anchored on either side in the area of the bead and is surmounted radially by a crown reinforcement made up of at least two layers that are superimposed and formed of threads or cords which are parallel in each layer and crossed from one layer to the next, forming angles of between 10° and 45° with the circumferential direction. The said working layers that form the working reinforcement may furthermore be covered with at least one layer, referred to as a protective layer, formed of reinforcing elements which are advantageously metallic and extensible and referred to as elastic reinforcing elements. It may also comprise a layer of metal threads or cords having low extensibility, forming an angle of between 45° and 90° with the circumferential direction, this ply, referred to as the triangulation ply, being located radially between the carcass reinforcement and the first crown ply, referred to as the working ply, formed of parallel threads or cords lying at angles not exceeding 45° in terms of absolute value. The triangulation ply forms a triangulated reinforcement with at least the said working ply, this reinforcement having low deformation under the various stresses which it undergoes, the triangulation ply essentially serving to absorb the transverse compressive forces that act on all the reinforcing elements in the crown area of the tire.
Cords are said to be inextensible when the said cords exhibit, under a tensile force equal to 10% of the breaking force, a relative elongation at most equal to 0.2%.
Cords are said to be elastic when said cords exhibit, under a tensile force equal to the breaking load, a relative elongation at least equal to 3% with a maximum tangent modulus of less than 150 GPa.
Circumferential reinforcing elements are reinforcing elements which make with the circumferential direction angles in the range +2.5°, −2.5° about 0°.
The circumferential direction of the tire, or longitudinal direction, is the direction that corresponds to the periphery of the tire and is defined by the direction in which the tire runs.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is a direction that intersects the axis of rotation of the tire and is perpendicular thereto.
The axis of rotation of the tire is the axis about which it turns in normal use.
A radial or meridian plane is a plane which contains the axis of rotation of the tire.
The circumferential median plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.
The “elastic modulus” of a rubber compound is to be understood as meaning a secant extension modulus at 10% deformation and at ambient temperature.
As far as rubber compositions are concerned, modulus measurements are taken under tension in accordance with standard AFNOR-NFT-46002 of September 1988: the nominal secant modulus (or apparent stress, in MPa) at 10% elongation (normal temperature and relative humidity conditions in accordance with standard AFNOR-NFT-40101 of December 1979) is measured in second elongation (i.e. after an accommodation cycle).
Some current tires, referred to as “road” tires, are intended to run at high speed and over increasingly long journeys, as a result of the improvement in the road network and of the growth of the motorway network throughout the world. The combined conditions under which such a tire is called upon to run without any doubt make possible an increase in the distance travelled, the wear on the tire being reduced, but on the other hand, the endurance of the tire and in particular of the crown reinforcement is detrimentally affected.
This is because there are stresses in the crown reinforcement and, more particularly, shear stresses between the crown layers, combined with a not-insignificant rise in the operating temperature at the ends of the axially shortest crown layer which have the effect of causing cracks in the rubber to appear and spread at the said ends.
In order to improve the endurance of the crown reinforcement of the type of tire under consideration, solutions relating to the structure and quality of the layers and/or the profiled elements of rubber compounds which are placed between and/or around the ends of the plies and, more particularly, the ends of the axially shortest ply have already been applied.
It is notably known practice to introduce a layer of rubber compound between the ends of the working layers in order to create an uncoupling between the said ends in order to limit shear stresses. Such decoupling layers must, however, exhibit a very good cohesion. Such layers of rubber compounds are, for example, described in Patent Application WO 2004/076204.
Patent FR 1 389 428, in order to improve the resistance to degradation of the rubber compounds situated near the crown reinforcement edges, recommends the use, in combination with a low-hysteresis tread, of a rubber profiled element covering at least the sides and the marginal edges of the crown reinforcement and made up of a low-hysteresis rubber compound.
Patent FR 2 222 232, in order to avoid separations between crown reinforcement plies, teaches the coating of the reinforcement ends in a cushion of rubber of Shore A hardness different from that of the tread surmounting the said reinforcement, and higher than the Shore A hardness of the profiled element of rubber compound placed between the edges of crown reinforcement and carcass reinforcement plies.
Tires produced in this way do effectively allow an improvement in performance notably in terms of endurance.
Moreover, it is known practice, in order to produce tires with a very wide tread or in order to confer greater load bearing capacity on tires of a given dimension, to introduce a layer of circumferential reinforcing elements. Patent application WO 99/24269 describes, for example, the presence of such a layer of circumferential reinforcing elements.
The layer of circumferential reinforcing elements is usually made up of a metal cord wound to form a turn of which the angle of lay with respect to the circumferential direction is less than 2.5°.
During testing, the inventors demonstrated that, when certain designs of this type of tire, comprising a layer of circumferential reinforcing elements, were loaded heavily and frequently in a manner such as may be encountered when driving along twisty roads, the said tires could exhibit properties in terms of cornering stiffness that are such that cleavage occurs at the rubber compounds of which the crown reinforcement found between the working layers is made. Such cleavage is, of course, penalizing in terms of the endurance properties of the tire. Such cleavage is further accentuated when running with overload and at high speed, again on twisty roads.